Chromizing and priming employing chromium and anhydrous magnesium chloride



March 26, 1968 A F (KILOCALORIES /MOLE) A. BALD! ETAL 3,375,128 CHROMIZING AND PRIMING EMPLOYING CHROMIUM AND ANHYDROUS MAGNESIUM CHLORIDE Filed Sept. 2, 1965 AF V- TEMPERATURE I08 KILOCAL/YMOLE |;so KlLOCAL/MOLE -5| KILOCAL/MOLE IPRIMING TEMP.

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1/j fiwO Z- %9 BY )VzZ/M %fe ATTORNEYS United States Patent O CHROMIZING AND PRIMING EMPLOY- ING CHROMIUM AND ANHYDROUS MAGNESIUM CHLORIDE Alfonso L. Baldi, Drexel Hill, and William E. Nice, Chadds Ford, Pa., assignors to Alloy Surfaces Company, Wilmington, Del., a corporation of Delaware Filed Sept. 2, 1965, Ser. No. 484,711 8 Claims. (Cl. 117107.2)

ABSTRACT OF THE DISCLOSURE The present invention relates to priming a porous metal source of chromium for chromizing by anhydrous magnesium chloride, to chromizing accomplished by such primed porous metal source of chromium and to a chromizing retort for usein such priming and chromizing.

DESCRIPTION OF INVENTION The present invention relates to priming of porous metal sources of chromium for use in chromizing and to processes and apparatus for chromizing iron or steel work.

A purpose of the invention is to prime a porous source of chromium such as ferrochrome by anhydrous magnesium chloride.

A further purpose is to obtain thicker chromized cases under given conditions by chromizing with a porous source of chromium which has been primed with anhydrous magnesium chloride.

A further purpose is to provide smoother chromized cases on iron and steel work, particularly titanium bearing steel, by chromizing with a porous source of chromium primed by anhydrous magnesium chloride.

A further purpose is to reduce the tendency of porous ferrochrome particles or granules to stick to ferrous metal work in chromizing.

A further purpose is to reduce the cost of priming a porous source of chromium for chromizing.

Further purposes appear in the specification and in the claims.

The present invention is a further development and improvement of the subject matter of Samuel, Bell, Grosvenor and Jackson US. Patent No. 3,021,231 for Chromizing. In order to simplify the specification and drawings, it is indicated that the mechanism shown in the drawings of this patent is suitable for use in the present invention and the general description and examples of said patent are applicable to the present invention except in respect to difierences in the priming agent, and improved results obtained in the present invention.

. As well known in the art, chromizing involves removing chromium from a source of chromium and depositing it on and impregnating it into the work, which in the case of the present invention, will be iron or steel.

In the technique of Patent No. 3,021,231 aforesaid, the source chromium is porous. While itmay be a powder metallurgy compact of chromium particles, it is decidedly preferable to use a powder metallurgy porous compact of ferrochrome.

In any case the source of chromium contains at least 30% chromium, the balance being iron except for impurities in the usual case. The ferrochrome will usually contain at least 65% of chromium by weight. Low carbon grade is decidedly preferable. The porous particles have a porosity of to 60% by volume, and preferably to by volume, and the pore space is interconnected so that gas can flow through it from one pore to another.

The source of chromium is in the form of granules or particles of a particle size between 0.02 and 3.00 inches 3,375,128 Patented Mar. 26, 1968 and preferably between 0.06 and 0.50 inch so that gas can circulate among the particles.

The priming is carried out in a retort in the presence of between 0.25 and 3% by weight of a mixture of anhydrous magnesium chloride at a temperature of 1650 to 2400 F. at a time of at least 30 minutes.

The chromizing itself is carried out in a retort in an atmosphere protected from the air at a temperature of 1650 to 2100 F. for a time of at least 30 minutes.

In order to effectively chromize using a porous source of chromium of this character, the source of chromium must be primed with a halide which will react with the chromium to deposit chromous halide on and in the pores of the source of chromium.

Various priming agents have been used as explained in the prior patent. Until the present invention, it has been considered that the most desirable priming agent for porous chromium or ferrochrome is chromic chloride, which reacts with the source of chromium to form chromous chloride and deposit it on and in the pores of the source of chromium.

Several difficulties have developed in connection with priming by chromic chloride.

It has been found that chromic chloride tends to produce a rather rough chromized case and also where it is in contact with the work tends to adhere rather firmly to the surface of decarburized low carbon or titanium stabilized low carbon steel work, adding greatly to the cost of chromizing by requiring labor to remove the ferrochrome particles from adherence to the work.

In the second place, chromic chloride is quite expensive, costing from $2 to $5 per pound.

In the third place, the volume of sales of chromic chloride in the American market has been so low, and the difiiculty with corrosion of apparatus during production of chromic chloride has been so great that the two American producers of chromic chloride have ceased to manufacture it.

Patent No. 3,021,231 aforesaid refers to a number of other priming agents. Seemingly most promising are the hydrated metal halides referred to in this patent, one of which is hydrated magnesium chloride, MgCl '6H O. This has the great advantage that on heating it breaks down into hydrogen chloride gas, and magnesium oxide, and hydrogen chloride is very effective to prime the source source of chromium.

On further experiment, however, it has been established that this results in the oxidation of the source of chromium. At moderate temperatures the magnesium chloride hexahydrate loses 5 moles of Water thus:

This water oxidizes the ferrochrome.

At higher temperatures the magnesium chloride monohydrate breaks down to form magnesium oxide thus:

Accordingly, a great deal of time in the next heat is used in reducing this oxidation, cutting down on the thickness of the case, and reducing the chromizing potential. Therefore, although priming with hydrous magnesium chloride is very economical, it greatly lengthens the time of a heat and reduces the elfectiveness of chromizing.

Anhydrous aluminum chloride was tried as a priming agent for the porous source of chromium and in fact aluminum chloride in small quantities of one to two grams per retort is currently used to sweep out the air and form a protective atmosphere during chromizing. When, however, aluminum chloride was employed in quantities great enough to prime the source of chromium, it was found that it clogged the vents because of its low sublimation temperature (350 F.) and greatly interfered with effective operation of the equipment, so that it is not a practical priming agent for the porous source of chrmium.

Cadmium bromide was tried as a priming agent. It tended, however, to poison the source of chromium by depositing a cadmium layer which interfered with further chromizing, and it tended to embrittle the chromized case.

Experiments with various bromides as priming agents have shown that they are generally hygroscopic and when the retort is open they absorb moisture from the air so that time is lost and chromizing potential is impaired on the next heat due to the presence of this moisture.

Barium fluoride was tried as a priming agent for the porous source of chromium, but it was found that it greatly diminished the thickness of the chromized case.

We have discovered that anhydrous magnesium chloride offers wholly unobvious advantages as a priming agent for a porous source of chromium. Magnesium chloride has previously been suggested for use as a catalyst in chromizing by pack methods using dense solid ferrochrome (Marshall US. Patent 1,853,369 and Bryant US. Patent 2,403,706), but no one has previously recognized the unobvious advantages which anhydrous magnesium chloride has for priming a porous source of chromium such as porous ferrochrome. It is not clear in the prior patents whether hydrous or anhydrous magnesium chloride is referred to.

We have discovered that when a porous source of chromium like porous chromium or ferrochrome is primed with anhydrous magnesium chloride, the case thickness obtained on iron or steel work under given conditions is at least to thicker than that obtained by the best prior priming agent, chromic chloride. This represents a great advantage in production, since it enables the operator to cut the time or reduce the temperature.

Furthermore, although this is not infallible, we have found that the porous source of chromium primed with anhydrous magnesium chloride is much more likely to produce a smooth chromized deposit than a source of porous chromium primed by any other known agent.

Accompanying this is the feature that a porous source of chromium primed with anhydrous magnesium chloride and coming into contact with iron or steel work is much less likely to stick to the work than when other priming agents are used, notably chromic chloride.

Anhydrous magnesium chloride is relatively low in cost, costing about 18 cents per pound as compared with chromic chloride which costs at least $2.00 per pound.

While a theoretical explanation of operation of hydrous magnesium chloride is readily available, since it forms hydrogen chloride gas, no clear-cut theoretical explanation as to how anhydrous magnesium chloride can react with a porous source of chromium to deposit chromous chloride is available. Free energy data suggest that this would not occur, and certainly it would not be anticipated that metallic magnesium would form from anhydrous magnesium chloride by reaction with porous chromium.

Example 1 In all of the examples, until further indication, the reaction was carried out in a retort as shown in FIGURE 1 of Patent 3,021,231 using various types of work, the ferrochrome having 70% chromium and low carbon, with a particle size of below 4 inch and above inch, the porosity in the ferrochrome being about 26% by volume. The ferrochrome charge per heat was approximately 30 pounds. The work and the ferrochrome were placed in the closed retort with a priming agent to be described and with one to two grams of anhydrous aluminum chloride as a purger. Following the technique described in Patent 3,021,231 the retort and contents were heated to and held at 1850 F. for five hours.

In each. of the experiments being described the work included the following:

(1) Titanium bearing steel tubing of approximately 1 inch OD, 7 inch ID, having an analysis of 0.30% titanium, 0.05% carbon, 0.28% manganese, 0.02% silicon and the balance usual metalloid impurities and iron. The titanium content is normally 5 to 10 times the carbon content.

(2) Titanium bearing steel bar of the same analysis, of /2 inch diameter and 3 inches length.

(3) A181 1010 steel sheet.

(4) Decarburized low carbon steel sheet having a carbon content of less than 0.003%, balance manganese, silicon, usual metalloids as above, plus iron.

(5) A181 1070 steel sheet.

In each of the experiments described, fresh porous ferrochrome was employed which had never before been used in chromizing.

The first experiment employed 1% of anhydrous calcium chloride mixed with the ferrochrome for priming. The work was put in at the same time.

The results were as follows:

(1) On the titanium bearing steel tube, the case was 2.0 mils thick, and the surface was rough.

(2) On the titanium bearing steel bar, the case was 2.0 mils thick and the surface was rough.

(3) An A181 1010 steel sheet the case was 2.2 mils thick and the surface was smooth.

(4) On the decarburized low carbon steel sheet the case was 1.5 mils thick and the surface was rough.

(5) On the A181 1070 steel sheet the case was 1.0 mil thick and the surface was smooth.

Example 2 The procedure of Example 1 was followed, except that 1% by weight of anhydrous barium fluoride was used to prime.

The result was that no measurable case whatever was obtained on any of the specimens.

Example 3 The procedure of Example 1 was carried out, except that 1% by weight of anhydrous barium chloride was used to prime.

The results were as follows:

(1) On the titanium bearing steel tube the chromized case was 1.5 mils thick and the surface was rough.

(2) On the titanium bearing steel bar the case was 1.0 mil thick and the surface was rough.

(3) No A181 1010 steel specimen was used.

'(4) On the decarburized low carbon steel sheet the chromized case was 1.2 mils thick and the surface was rough.

(5) No AISI 1070 steel specimen was included.

Example 4 The procedure of Example 1 was followed, except that 1% by weight of anhydrous sodium chloride was used to prime the ferrochrome.

The results were as follows:

(1) On the titanium bearing steel tube the chormized case was rough and had a thickness of 1.8 mils.

(2) On the titanium bearing steel bar the chromized case was rough and had a thickness of 1.8 mils.

(3) On the A181 1010 steel sheet the chromized case was rough and had a thickness of 1.8 mils.

(4) On the decarburized low carbon steel sheet the chromized case was rough and had a thickness of 2.10 mils.

(5) No A181 1070 steel specimen was used.

Example 5 The procedure of Example 1 was followed, except that 1% by weight of anhydrous chromic chloride was used to prime the ferrochrome.

a The results were as follows:

(1) The titanium bearing steel tube had a rough chrd mized case and the thickness was 2.0 mils.

(2) The titanium bearing steel bar had a smooth chromized case and the thickness was 2.5 mils.

(3) The AISI 1010 steel sheet had a smooth chromized case and the thickness was 1.8 mils.

(4) The decarburized low carbon steel sheet had a rough chromized case and the thickness was 1.5 mils.

(5) The AISI 1070 steel sheet had a smooth chromized case and the thickness was 1.0 mil.

Example 6 The procedure of Example 1 was carried out, except that the porous ferrochrome was primed with 1% by weight of anhydrous magnesium chloride.

The results were as follows: (1) The titanium bearing steel tube had a smooth chromized case with a thickness of 2.2 mils.

(2) The titanium bearing steel bar had a smooth case with a thickness of 3.0 mils.

I (3) The AISI 1010 steel sheet had a smooth chromized case with a thickness of 2.5 mils.

(4) The decarburized low carbon steel sheet had a rough chromized case with a thickness of 2.0 mils.

(5) The AISI 1070 steel sheet had a smooth case with a thickness of 1.1 mils.

Example 7 As a control, unprimed porous ferrochrome was heated under the conditions of Example 1 using only titanium bearing steel tubing and AISI 1010 steel sheet as specimens.

The chromized case on the titanium bearing steel tube had a thickness of 0.5 mil. No case whatever formed on the AISI 1010 steel sheet.

Example 8 The procedure of Example 1 was carried out using 1% by .weight of anhydrous magnesium chloride as a primer, and the heat was run with no work whatever in the retort. One to two grams of aluminum chloride was added as a purger. The chromous chloride formed on and in the pores of the porous ferrochrome was then extracted and determined. It was found that the priming with extractable chromous chloride amounted to 500 grams per hundred pounds of ferrochrome.

Example 9 The procedure of Example ,9 was repeated. again with thesame ferrochrome, adding miscellaneous steel samples for chromizing as above referred to.

The results obtained for the chromized case were com parable'to those above stated, and the priming of extractable chromous chloride was found to be 635 grams per hundred pounds of primed ferrochrome. The increase is more than can be accounted for by reaction with the small amount of aluminumchloride and is believed to be due to further completion of the priming reaction whic made more chromous chloride available.

Example 11 The procedure of Example 9 was repeated with the same ferrochrome and again a sample of the primed ferrochrome was analyzed at the end of the heat. It was found that the extractable priming of chromous chloride amounted to 630 grams per hundred pounds of primed ferrochrome.

As set forth in Patent 3,021,231, the range of priming of chromous chloride should be kept between 300 and 12-00 grams per hundred pounds of primed ferrochrome or other porous sources of chromium.

Example 12 A series of experiments was run to compare the effect as primers for porous ferrochrome, of hydrous magnesium chloride, anhydrous magnesium chloride, and chromic chloride.

In the first experiment, 1.6% by weight of the mixture of hydrous magnesium chloride (the hexahydrate) was blended with fresh porous ferrochrome as above referred to and one to two grams of aluminum chloride was added as a purger. The retort was closed and heated to 1800 F. for five hours.

After cooling to room temperature the retort was opened and the ferrochrome particles were examined in detail. It was found that approximately 50% of the ferrochrome particles were badly discolored due to reaction with moisture evolved from the hydrous magnesium chloride.

Example 13 The procedurev of Example 12 was carried out using fresh porous ferrochrome, except that instead of using hydrous magnesium chloride, 1% of anhydrous magnesium chloride was incorporated with the porous ferrochrome and heating carried on as in Example 12. At the end of the run the ferrochrome particles were examined in detail. The ferrochrome was found to be bright in appearance indicating that no oxidation had occurred.

Example 14 The procedure of Example 13 was carried out with fresh porous ferrochrome, except that in this case 1% by weight of the mixture of anhydrous chromic chloride was used as a primer. The heating was carried on as before and at the end of the run the ferrochrome was examined in detail. The particles were found to be bright in appearance indicating absence of oxidation.

Example 15 Samples of the following types of steel were placed in the retort with the chromizing pack primed with chromic chloride as in Example 14:

1) Decarburized AISI 1010 steel having a carbon content of 0.003%.

(2) AISI 1010 steel sheet.

3) Titanium bearing low carbon steel having 0.30% titanium, 0.05% carbon, and the balance standard metalloids as above.

The chromizing was carried on at 1850 F. for five hours. The case depths were measured by grinding away the edge and dissolving the base metal in 25% by weight hot nitric acid in water.

The results were as follows:

(1) On the decarburized steel sheet the chromized case was ductile but very rough and had a thickness of 1.6 mils.

(2) On the AISI 1010 steel sheet the chromized case Wals ductile but slightly rough and had a thickness of 1.5 m s.

(3) On the titanium bearing steel sheet the chromized case was ductile and had a fine roughness throughout and a thickness of 1.9' mils.

Example 16 The procedure of Example 15 was carried out, using, however, porous ferrochrome primed with 1% anhydrous magnesium chloride in Example 13. The results were as follows:

(1) On the decarburized steel the chromized case wa ductile and moderately rough, and had a thickness of 1.8 mils.

(2) On the A181 1010 steel sheet the chromized case was ductile, had a uniform smooth surface and a thickness of 2.1 mils.

(3) On the titanium bearing low carbon steel sheet the chromized case was ductile, uniform and smooth throughout and had a thickness of 2.3 mils.

It will be noted by comparing the results in Example and Example 16 that the case depths in Example 16 are markedly greater where anhydrous magnesium chloride was used to prime the porous ferrochrome than in the previous example where anhydrous chromic chloride was used.

Example 17 In commercial pack chromizing 50,000 pounds of fresh porous ferrochrome has been primed with anhydrous magnesium chloride (1%). Large tonn'ages of steel, of both plain carbon and titanium bearing grades, have been chromized using this ferrochrome. It has been found that for a reason which is not apparent the greater ease depths at a given temperature and a given time and greater tendency to form smooth deposits as previously explained in connection with Examples 15 and 16, has prevailed through this commercial production. Because of this greater case depth, temperatures of chromizing have been lowered to of the order of 1800 F., obtaining results similar to this previously obtained at 1850 F. in costs are required in removing the work from the furnaces.

Furthermore, it has been observed that far less sticking of ferrochrome to the work occurs and lower labor the same time and with less wear and tear on the furnace and cleaning it.

Due to the fact that the deposit is inherently smooth, labor costs are also saved because removal of sintered adherent ferrochrome particles from the work is not necessary.

The figure shows as ordinates the free energy of formation of the compounds in kilocalories per mole and as abscissae degrees Fahrenheit.

It will be noted that from the free energy data for ferric chloride it would be expected that ferric chloride would be an effective primer to form chromous chloride in the pores of the source of chromium. On the other hand, these data would suggest that anhydrous magnesium chloride would not participate in such a reaction and it is therefore not possible at this time to offer a complete explanation of why these results occurred.

It is difficult to postulate a theory since the thermodynamic data suggest that in a system consisting merely of anhydrous magnesium chloride, MgCl metallic chromium and metallic iron, the reaction to form chromous chloride is metallurgically impossible.

One possible theoretical explanation may be that at the temperature of chromizing, anhydrous magnesium chloride breaks down to form magnesium mono-chloride plus chlorine, and the chlorine reacts with chromium to form chromous chloride.

Another possible explanation, which may actually cooperate with the first explanation, is that the ferrochrome or other sources of chromium, having a large number of pores, tends to adsorb moisture when it is exposed to the atmosphere at the end of a heat. Although this moisture does not make the primed ferrochrome hydroscopic in the sense that its surface indicates presence of moisture, it may pick up moisture in the pores. In fact, the porous ferrochrome as sold may inherently contain adhering moisture in its pores. When this ferrochrome is heated in the retort with anhydrous magnesium chloride, as the temperature rises, aluminum chloride vaporizes and teniis to expel air and moisture from the retort. At the same time, however, anhydrous magnesium chloride being a powerful dehydrating agent, is competing to adsorb moisture in minute amounts. At higher temperatures this adsorbed moisture can react with the magnesium chloride to form magnesium oxide and hydrogen chloride which is capable of reacting with the chromium to form chromous chloride. This chromous chloride, having a high vapor pressure at chromizing temperature, brings chromium to the work and reacts with thework to deposit chromium and pick up iron, forming ferrous chloride which, in turn, reacts with chromium in the, ferrochrome to form more chromous chloride, and deposits iron. Unlike the reaction obtained with hydrous magnesium chloride, which oxidizes the ferrochrome and reduces the chromizing potential, this reaction involving minute amounts of water, does not oxidize the ferrochrome and does not lower the chromizing potential, thus producing unexpectedly thick cases in a given time and temperature.

These explanations are offered with great caution and the validity of any patent granted is not predicated upon the correctness of the theory.

Even though it were possible to explain the effectiveness of magnesium chloride as a primer of porous chromium and ferrochrome, it does not seem possible to explain from present data why, as the facts appear, anhydrous magnesium chloride primed porous ferrochrome gives more deposit in a given time and gives a smoother deposit than ferrochrome primed, for example, with anhydrous chromic chloride.

For general details of the process, the reader is referred to the prior patent. Thus he will find in the prior patent discussion of purging the retort, and the arrangement and operation of the apparatus. It will also be understood that as in the prior patent the invention is applicable to porous powder metal compacts of chromium, as well as ferrochrome.

In view of our invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of our invention without copying the process and apparatus shown, and we therefore claim all such insofar as they fall within the reasonable spirit and scope of our claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A process of priming a porous metal source of chromium for chromizing, which comprises reacting within a retort a porous metal source of chromium containing at least 30% by weight of chromium, having a particle size between 0.02 and 3.0 inches and having an interconnected porosity of 5 to 60% by volume, with between 0.025 and 3% by weight of the mixture of anhydrous magnesium chloride at a temperature of l650 to 2400 F. for a time of at least 30 minutes in an atmosphere protected from the air.

2. A process of claim 1, in which the source of chromium is ferrochrome.

3. A process of claim 1, in which the concentration of anhydrous magnesium chloride is about 1% by weight.

4. A process of chromizing iron or steel work to make thicker cases in a given time under given conditions, which comprises reacting within a retort a porous metal source of chromium containing at least 30% by weight of chromium, having a particle size between 0.02 and 3.00 inches and having an interconnected porosity of 5 to 60% by volume, with between 0.25 and 3% by'weight of the mixture of anhydrous magnesium chloride at a temperature of 1650 to 2400" F. for a time of at least 30 minutes in an atmosphere protected from the air, to prime the source of chromium, heating iron or steel work in a retort with said primed source of chromium in an atmosphere protected from the air to a temperature of 1650 to 2100 F. for at least 30 minutes, and allowing said retort and its contents to cool.

5. A process of claim 4, in which the source of chromium is ferrochrome.

6. A process of claim 4, in which the concentration I hydrous magnesium chloride for priming the source of of anhydrous magnesium chloride is about 1% by weight. h o i 7. A process of claim 4, in which the work is titanium bearing steel. References Cited 8. A chromizing retort comprising a sealed container 5 UNITED STATES PATENTS closed to gas flow from the outside adapted to be placed 2 403 706 7/1946 Bryant 117 22 in a furnace and adapted to contain 1ron or steel work, 3,021,231 2/1962 Samuel et al- 1 7 1 a porous metal source of chromium within the retort containing at least 30% by weight of chromium, having a particle size between 0.02 and 3.00 inches and having 10 ALFRED LEAVITI" Pnmary Exammer' 'an interconnected porosity of 5 to 60% by volume, and A. GOLIAN, Assistant Examiner.

between 0.25 and 3% by weight of the mixture of an- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,375,128 March 26, 1968 Alfonso L. Baldi et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 46, strike out "source", second occurrence;

' column 4, line 27, for "An AISI" read On the AISI line 61, for "chormiz'ed" read chromized column 7, line 30, Strike out "costs are required in removing the work from the furnaces" and insert instead the same time and with less wear and tear on the furnaces lines 33 and 34, strike out "the same time and with less wear and tear on the furnace and cleaning it and insert instead costs are required in removing the work from the furnace and cleaning it Signed and sealed this 29th day of July 1969.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER,JR. Attesting Officer Commissioner of Patents 

